They are scattered all over. some are on the new digital business trunking systems. (DMR, NXDN)
The Sunshine LTR's are your best bet to try. (Since they are conventional)
75 percent of the companies are on the sunshine system. Yellow cab uses encrypting on a icom idas system. City cab uses a 220mhz trunk system. But keep in mind they use mdt computers city wide for most dispatching.